Grout ℞: Repair a Leaking Tiled Shower Floor

Dealing with leaky shower pans, mildew, mold, and rotting wood

Homeowners can face a number of stubborn issues regarding tile shower floors that range from the cosmetic issues to water damage. Until recently, almost all tile floors were built on mortar beds, and to this day this method of construction is a preferred by many seasoned tile setters. If built correctly, a shower floor built over a mortar bed can be long lasting and reliable; however, the complexity of the process can cause many issues down the road if several key steps are not properly executed. Even if such floors are properly installed, they often become breeding grounds for mold that can be difficult, if not impossible to get rid of.

Here we explain the most common issues and what we do to repair them with long-lasting results.

Cutaway illustration of shower construction

Anatomy of a tiled shower floor

If we were to strip everything out of your bathroom down to the 2x4 studs and the ¾ inch plywood substrate and build a shower from scratch employing the mortar bed method, we would first start by installing a foam core pre-slope. This pre-slope is installed to encourage water that gets through the grout joints to head toward the drain. A PVC cloth liner is then installed that covers the area of the shower floor including the threshold and goes about 6 inches up the studs. This liner acts a waterproof barrier or a tub of sorts. At the point where the drain pipe meets the liner, it screws into a threaded pipe and fits firmly over top of the liner so as not to let any water through. There are weep holes at this junction that head into the drain pipe that allow water to drain after it soaks into the mortar bed.

After the vinyl liner is installed a “mud” or mortar bed is constructed to provide slope to the drain. Tile is then thin-set over the mortar bed, allowed to dry, then grouted. Most grout and thin-set products are Portland based cements that are at best semi porous even when sealed. They let water into the mortar bed that absorbs it willingly. As gravity encourages the water to seek the lowest point, the water slowly come to settle on the bottom of the liner where, if the pre-slope was installed properly underneath the liner, the water proceeds to travel down to the weep holes. If the weep holes were properly covered with pea-gravel or a protective collar to prevent clogging, the water then drains into the drain pipe through what is commonly referred to as the secondary drain.

Issues over time

Over time and in cases where the shower is frequently used, wear and tear begin to show in the grout and caulk which do wear out. Grout should be resealed every few years to maintain the integrity of the shower.

Where materials break down, mold invariably begins to grow and penetrates the mortar bed where it thrives in an area where both water and organic material are abundant (the two things mold needs to grow).

When the homeowner goes to clean the shower floor, often times they cannot scrub out the mold or if it goes away, it returns quickly as the source can never be reached. This is similar to pulling weeds and only getting the top and not the roots – the problem grows right back.

Leaking shower resulting in mold and mildew and rotted wood


Efflorescence built up in and around a tile shower drain

Another issue called efflorescence can occur if the weep holes to the secondary drain are blocked (usually from improper installation.

If water cannot drain through the mortar bed, it collects. As it tries to evaporate, the water leaves mineral deposits on the grout lines that look like stalagmites or miniature mountains and are crystalline in nature.

This is largely a cosmetic problem but suggests improper construction and can lead to the slow degradation of the mortar bed.

Improper Installation

An improperly installed liner can also let water leak through. As water traverses the mortar bed, it eventually settles on the liner where it heads toward the secondary drain. If there are any holes in the liner or the seams are improperly joined, water can leak through and get into the wood substrate causing rot, mold, and other issues related to water damage.

This issue can appear years after a shower is installed and is sometimes very difficult to detect.

A sure fire way of testing a shower floor to see if it leaks is to place a drain stop in the shower drain and fill the pan up with water to the level of where the wall starts. If the water sits for 10 hours without a leak appearing underneath the shower or the ceiling below, it is likely that the liner is intact and is protecting your house from water damage.

failing tiled shower pan before repair

Repairs and Remedies for Leaking Shower Pans

All of the above issues can be remedied by the installation of a shower floor overlay.
2 days to build, 7-14 days to cure

Setting up new tiles and drain onto shower pan overlay to repair leaks

If the slope of an existing shower floor allows positive drainage, an existing shower floor can be allowed to dry for a few days and then be used as a template for a floor overlay. A coating of marine epoxy resin is bonded to the whole shower floor, effectively sealing the surface from any more water penetration.

The drain screen is removed from the drain and a ¼ inch PVC drain raise is affixed to the current drain pipe. Tile is then set with another resin application and allowed to dry for 24 hours. The tile is then grouted with an epoxy grout called Spectra Lock (a Laticrete product).

Once the floor cures, there are in essence two layers of waterproofing that don’t let water enter the mortar bed – the resin coating and the tile and epoxy grout.

We usually grout up the tile walls one row up with the Spectra Lock to ensure a waterproof pan.

Tiled walk-in shower pan with new overlay and drain

Once this process is complete, it takes 14 days for the epoxy grout to completely cure – 7 days if temperatures are above 80 degrees Fahrenheit.

When the curing is complete, the homeowner has a waterproof floor.

All the water goes into the drain and no mold can penetrate into the grout or a mortar bed. Any mold that might occur due to lack of regular preventative maintenance can be scrubbed right off and never grows beneath the surface. Leaks are prevented and no efflorescence builds up as there is no way the water has access to the old mortar bed.

All in all the process takes two days to build and raises the height of the floor just over a ¼ of an inch.

Two days to build and 7-14 days to cure give you long-lasting results and improve your property.

Grout Rx can also convert a traditional shower, toilet, or bathroom into a wet room, with the shower floor level with the rest of the room. The entire room can be waterproofed, and may be made barrier-free and even wheelchair accessible if there is enough space.